Converting 1/4-20unc thread to 1/4-20 whitworth
Close
Login to Your Account
Results 1 to 9 of 9
  1. #1
    Join Date
    Jul 2019
    Country
    CANADA
    State/Province
    British Columbia
    Posts
    27
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1

    Default Converting 1/4-20unc thread to 1/4-20 whitworth

    I bought a set of collets for a very small machine i have, but for some reason, the drawbar uses 1/4-20 whitworth thread. A regular 1/4-20 UNC does not fit more than a few turns of the threads into the collets, so i cannot just directly use a 1/4-20 bolt.


    Is there a fast way to modify a 1/4-20 bolt into 1/4-20 whitworth? I thought id just be able to run a 1/4-20 whitworth die over a 1/4-20 bolt, and this would remove the material that is the difference between the threadforms. I thought this would work, since the 55 degree thread form should have less metal, and the die should cut into the 1/4-20 UNC bolt somewhat. But this was not the case, perhaps the die i have is worn, but i can freely thread a 1/4-20 UNC bolt thru my 1/4-20 whitworth die. In fact it kind of feels loose in there, which makes no sense to me, i would expect it only to be loose the other way around, if i were to put a whitworth bolt, into a UNC nut for instance.


    Of course i could turn the dang thing myself, but due to clearance/reach issues, i would far prefer the end of the bolt to have a hex socket, and this is not a feature i can readily make. In fact right now i dont even have access to a lathe that can thread.


    I thought it would be as easy as just chasing the UNC bolt with the whitworth die, and the 5 degrees of extra material would be removed or chased off by the whitworth die.
    Does that fact that this did not happen indicate that my die is worn out? it is from an old set from 1986, but its a complete whitworth set i have lying around.

    Thank you.

  2. #2
    Join Date
    Aug 2014
    Location
    Wisconsin Rapids WI
    Posts
    528
    Post Thanks / Like
    Likes (Given)
    3
    Likes (Received)
    129

    Default

    A UNC bolt should fit into a BSW nut but the fit is loose. You have it backwards, a bsw bolt won't fit a UNC nut. If the UNC bolt doesn't fit the thread, what you have is different than what you think you have. Dave

  3. Likes muckalee liked this post
  4. #3
    Join Date
    Sep 2009
    Location
    Phoenix,AZ
    Posts
    2,122
    Post Thanks / Like
    Likes (Given)
    3426
    Likes (Received)
    583

    Default

    You might have a BA 0 thread which is very close to 1/4 -20 but a bit smaller as I recall. Smiths speedo/tachs on M/C used 0BA on the mounting studs.

  5. #4
    Join Date
    Nov 2003
    Country
    UNITED STATES
    State/Province
    Texas
    Posts
    6,146
    Post Thanks / Like
    Likes (Given)
    225
    Likes (Received)
    2124

    Default

    Something does not quite make sense here. If the 1/4-20 UNC bolt goes in a few threads but then jams, that seems to point to the thread form being loose enough to fit but the pitch being wrong. But you say they are both 20 TPI.

    What do you base the collet's thread being 1/4-20 Whitworth? Do you have a Whitworth bolt that fits? Or a tap? Do you have the drawbar and can see/measure the thread there? Or is the collet marked?

    OR, could the thread perhaps be something else, like metric? M6 x 1 or perhaps M6 x 1.25? This last one is very close as 20 TPI = a metric 1.27 pitch.

    I would coat that 1/2-20 bolt with DyKem, screw it in, wiggle it about a bit, and then see where the interference is. If the contact areas switch from one thread flank to the other, then you have a different pitch. If the contact is always at the top or bottom of the thread, then it could be the thread form (Unified 60° or Whitworth 55° or even something else).



    Quote Originally Posted by dzarren View Post
    I bought a set of collets for a very small machine i have, but for some reason, the drawbar uses 1/4-20 whitworth thread. A regular 1/4-20 UNC does not fit more than a few turns of the threads into the collets, so i cannot just directly use a 1/4-20 bolt.


    Is there a fast way to modify a 1/4-20 bolt into 1/4-20 whitworth? I thought id just be able to run a 1/4-20 whitworth die over a 1/4-20 bolt, and this would remove the material that is the difference between the threadforms. I thought this would work, since the 55 degree thread form should have less metal, and the die should cut into the 1/4-20 UNC bolt somewhat. But this was not the case, perhaps the die i have is worn, but i can freely thread a 1/4-20 UNC bolt thru my 1/4-20 whitworth die. In fact it kind of feels loose in there, which makes no sense to me, i would expect it only to be loose the other way around, if i were to put a whitworth bolt, into a UNC nut for instance.


    Of course i could turn the dang thing myself, but due to clearance/reach issues, i would far prefer the end of the bolt to have a hex socket, and this is not a feature i can readily make. In fact right now i dont even have access to a lathe that can thread.


    I thought it would be as easy as just chasing the UNC bolt with the whitworth die, and the 5 degrees of extra material would be removed or chased off by the whitworth die.
    Does that fact that this did not happen indicate that my die is worn out? it is from an old set from 1986, but its a complete whitworth set i have lying around.

    Thank you.

  6. #5
    Join Date
    Sep 2011
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    22,025
    Post Thanks / Like
    Likes (Given)
    7561
    Likes (Received)
    12685

    Default

    Quote Originally Posted by EPAIII View Post
    OR, could the thread perhaps be something else, like metric?
    ^^^ THIS ^^^. Has legs.

    Metric fasteners are not uncommon. Neither is metric tooling.

    Whitworth IS uncommon.

    Grab a Metric fastener. If not already in your own Hell Box, seek at an Auto supply or Big Box and see what actually fits.

    You may be trying to piss up the wrong rope.

  7. #6
    Join Date
    Jun 2012
    Location
    Michigan
    Posts
    6,267
    Post Thanks / Like
    Likes (Given)
    5862
    Likes (Received)
    4014

    Default

    Quote Originally Posted by dzarren View Post
    A regular 1/4-20 UNC does not fit more than a few turns of the threads into the collets
    This tells me that both are not 20tpi. One of them has a different pitch.
    Are you positive that one of them isn't an M6 x 1 thread?

  8. #7
    Join Date
    Aug 2014
    Location
    Wisconsin Rapids WI
    Posts
    528
    Post Thanks / Like
    Likes (Given)
    3
    Likes (Received)
    129

    Default

    It will be interesting to see if the thread is BA 0 or 6mm 1. My English machines with Whitworth threads never have Metric unless modified. The changegears and settings on my lathe are the same for BA 0 and metric 1.0. Dave

  9. #8
    Join Date
    Aug 2015
    Country
    UNITED STATES
    State/Province
    Illinois
    Posts
    1,753
    Post Thanks / Like
    Likes (Given)
    1370
    Likes (Received)
    749

    Default

    BA 0 and M6x1 have the same OD and pitch. BA has 47.5* included angle and rounded crest and root. M6 is 60* included and flats at crest and root.
    Whitworth has a 55* included angle.

  10. #9
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    6,592
    Post Thanks / Like
    Likes (Given)
    131
    Likes (Received)
    1272

    Default

    Just looking at the profiles of the two threads. UNF has a flat on top with straight side. The other has a rounded profile.
    Run a file along the thread grooves to round over the peaks. Might work if the peaks of the UNF are not the same height.
    The job done by hand would probably look good but the in-exactness of the hand work will show up if the threads are supposed
    to be self-sealing. A sloppy fit because 60 degrees != 55.


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •